Spectrophotometer Finds Cure for Edo Blues

Art historians studying the work of Japanese masters of ukiyo-e paintings had a problem: This style used very thin layers of paint. "It was impossible to get a fragment to work with," said Marco Leona, a scientist on the staff of the Freer Gallery of Art/Smithsonian Institution.

To solve this, Leona turned to a UV-VIS Cary 50 spectrophotometer manufactured by Varian Inc. and equipped with a custom-made tapered fiber optic probe from CIC Photonics. Leona explained: "The fiber optic probe allows us to get curves from the pigment without any destructive sampling. ... We can chemically identify a pigment without even taking the painting off the wall." The system can record a reflectance spectrum from 250 to 900 nm, giving the user an extended wavelength range in comparison with older spectroscopic methods.

By using this apparatus, researchers can look for the presence of pigments such as Prussian blue, or ferric ferrocyanide. Introduced by Westerners to Japan in 1763, this chemical compound -- originally made by combining iron salts and cattle blood -- produces a brilliant, deep blue. It was rapidly adopted by painters of ukiyo-e, which depicts the "floating world" of entertainers, geishas and courtesans during the Edo period (1615-1868), when Japan was a closed society just becoming aware of the West.

Japanese artists previously used a duller, grayer, plant-based pigment called indigo. Since indigo peaks sharply at 650 nm and does not absorb in the near-IR, while Prussian blue absorbs strongly from the near-IR into the UV, researchers now have an easier way of pinning down the identity of a blue pigment.

Traditional ukiyo-e artists such as Katsushika Hokusai, the Rembrandt of this style, were quick to use the brilliant new imported pigments from the West. Researchers examining the blue dots in his painting, "Portrait of a Courtesan Walking," found it contained one of the first examples of the use of Prussian blue. The red curve on the computer screen shows the presence of indigo; the blue curve shows Prussian blue. Courtesy of the Freer Gallery of Art/Smithsonian Institution.The presence or absence of Prussian blue in a painting has several implications for art history, Leona said. If an old Japanese painting contains Prussian blue, then researchers know that the painting cannot have been made before 1763. Similarly, researchers can use the presence of Prussian blue as a tag to track the spread of Western influences in Japan, and to see how and why painters adopted it.

The small, portable spectrophotometer also has the advantage of allowing researchers to work with the room lights on. Previously, scientists had to shield the painting from any stray light sources that would fool the detector. They could do this either by putting a small black cylinder around the first few inches of the probe and pushing it up against the painting, or by taking the painting into the laboratory and turning off the lights.

Because this new spectrophotometer contains a xenon flashlamp 10,000 times more powerful than any room light, stray ambient light is no longer a problem.